Research and development of iridium cerium photocathode for SuperKEKB injector linac

We present the results of our investigation on iridium cerium compound as a photocathode material for the SuperKEKB photoinjector. A large-size iridium cerium compound with excellent machinability was obtained by a two-stage production method combining cold crucible induction melting and spark plasma sintering, and its usability as photocathode material was verified. The quantum efficiency (QE) of the non-activated sample was measured with 266 nm laser pulses to be 2.12 × 10−6. The QE of the cleaned sample was found to be 1.49 × 10−4, exhibiting an improvement by a factor ~ 70. The high resistance to poisoning against oxidization and carbonization of iridium cerium compound leads to useful properties, such as the compound being easily activate by laser cleaning and its reasonably high QE being maintained under non-ultra-high vacuum conditions. These significant advantages of the iridium cerium photocathode allow for the generation of high-charge electron beams with a bunch charge of 4.8 nC, when used with an advanced radio-frequency (rf) gun in the SuperKEKB injector linac. The QE of iridium cerium compound which was mounted in the rf gun was found to stay within the range of (8 ~ 10) × 10−5 for a year and a half without cathode maintenance such as laser cleaning.